It has been conventional practice to produce mineral fibers, and particularly glass fibers, through electrically heated bushings. An electrically heated screen is positioned in the bushing for the flow of mineral material therethrough. Such conventional heater screens are used to thermally condition the material as it passes through the bushing. Such heater screens are conventionally very porous and are not used to regulate flow of a glass in the bushing. The ratio of the resistance to flow of the heater screen to the resistance to flow of the orificed bushing bottom wall is from about 0.05 to about 0.09.
In conventional bushings, the flow through the bushing is generally controlled by the orifices in the bottom wall. The throughput of the bushing is greatly affected and can be disrupted by fluctuations of power to the bottom wall of the bushing, thermal transients in the glass from the glass supply, and thermal transients from the environment beneath the bushing. Improved bushing design is desired for more positive control over bushing throughput.